Perpendicular magnetic recording system is adopted as a technique for increasing the magnetic recording density. A perpendicular magnetic recording medium at least comprises a substrate, and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic backing layer formed of a soft magnetic material and playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; a base layer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
Japanese Patent Laid-Open No. 2001-291230 discloses a granular magnetic material as a material for forming the magnetic recording layer in the perpendicular magnetic recording medium (see PTL1). The granular magnetic material comprises magnetic crystal grains and non-magnetic substance segregated to surround the magnetic crystal grains. Magnetic crystal grains within the granular magnetic material are magnetically separated from each other by the non-magnetic substance.
For the purpose of further increasing the recording density of perpendicular magnetic recording media, an urgent need for reduction in the grain diameters of the magnetic crystal grains arises in recent years. On the other hand, reduction in the grain diameters of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (signals). In order to compensate for the decline in thermal stability due to the reduction in the grain diameters of the magnetic crystal grains, the magnetic crystal grains in the granular magnetic material need to be formed of materials with higher magnetocrystalline anisotropies.
One of proposed materials having the required higher magnetocrystalline anisotropies is L10 ordered alloys. Japanese Patent Laid-Open No. 2004-178753 describes L10 ordered alloys containing at least one type of element selected from the group consisting of Fe, Co and Ni, and at least one type of element selected from the group consisting of Pt, Pd, Au and Ir, and the method for preparing the L10 ordered alloys (see PTL2). Typical L10 ordered alloys include FePt, CoPt, FePd, CoPd, and the like.
The use of ordered alloys as a magnetic material has been proposed in various fields. Japanese Patent Laid-Open No. 2003-296901 describes a spin memory type magnetic recording medium comprising a base layer, a magnetic recording layer including Co-based alloy or an ordered alloy, an intermediate layer, and a highly polarized spin controlling layer (see PTL3). Here, the ordered alloy may include Fe50Pt50, Fe50Pd50, and Co3Pt1. Further, there is a description that the magnetic properties may be improved by adding an additive element such as Cu, Cr, or Ti to the Co-based alloy or the ordered alloy which forms the magnetic recording layer. In addition, there is a description that the gaps between magnetic crystal grains formed of the Co-based alloy or the ordered alloy may be filled with a non-magnetic substance consisting of Cr, Ta, B, oxides, or nitrides. Further, there is a description that a Pt film can be used as the base layer for the magnetic recording layer consisting of FePt. However, Japanese Patent Laid-Open No. 2003-296901 does not suggest solutions of various problems which are needed to overcome when adopting the ordered alloys in HDD's, since the above patent literature relates to spin memory type magnetic recording. Further, the above patent literature does not teach or suggest the use of Ti as the non-magnetic substance for filling the gaps between the magnetic crystal grains formed of the ordered alloy.
On the other hand, reduction in the sizes of the magnetic crystal grains means reduction in the cross-sectional areas of the crystal magnetic grains having a certain height, since the thickness of the magnetic recording layer is basically uniform in an in-plane direction of the medium. Therefore, a diamagnetic field acting on the magnetic crystal grains themselves decreases whereas a magnetic field required reversing the magnetization of the magnetic crystal grains (magnetic switching field) increases. As described above, the improvement of the recording density implies that a larger magnetic field is required for recording signals, in view of the shape of the magnetic crystal grains,
In regard to the problem of increase in magnetic field strength required for signal recording, there is proposed a method for reversing the magnetization with weak magnetic field with the help of magnetic domain wall displacement. Japanese Patent Laid-Open No. 2005-285207 describes a process for producing a magnetic thin film comprising forming a FePt magnetic thin film by sputtering onto a substrate whose surface temperature is 650-850° C., wherein the magnetic thin film has a coercive force of 40 kOe (3200 A/mm), and magnetization can be reversed with a magnetic field of 4-10 kOe (320-800 A/mm) (see PTL4). The above patent literature discloses that a base layer consisting of MgO, ZnO, Cr or Pt may be disposed when glass is used as the substrate. In order to utilize the magnetic domain wall displacement, it is necessary that the magnetic thin film in this method is a discontiguous layer consisting of isolated island-like FePt particles. That is, the magnetic thin film does not have a structure in which a non-magnetic body is filled within gaps between the FePt particles.
Energy-assisted magnetic recording systems such as a heat-assisted recording system or a microwave-assisted recording system have been proposed as the other means against the problem of increase in the magnetic field strength required for signal recording (see NPL1). The heat-assisted recording system utilizes the temperature dependence of the magnetic anisotropy constant (Ku) of a magnetic material, which is a characteristic where the higher the temperature, the lower the Ku. This system uses a head having functions to heat a magnetic recording layer. In other words, this system executes writing while reducing a magnetic switching field by raising the temperature of the magnetic recording layer to temporarily reduce the Ku. The recorded signals (magnetization) can be maintained stably, since the Ku returns its original high value after the temperature of the magnetic recording layer drops. In the application of the heat-assisted system, a magnetic recording layer needs to be designed taking its temperature characteristics into consideration, in addition to the conventional design guidelines.